Kristina Nedenskov Jensen

Three-Way Modelling of NMR Relaxation Profiles from Thawed Cod Muscle

Abstract Low-field 1 H nuclear magnetic resonance transverse relaxation was used to measure water mobility and distribution in cod stored at −20°C or −30°C for up to 12 months and subsequently from 0 to 21 days in modified atmosphere at +2°C. The relaxation profiles were decomposed by parallel factor analysis resulting in four first-order relaxation curves from which the relative water pool sizes and the transverse relaxation times (T2) were calculated. The T2-values of the four identified water pools were 37 ms, 56 ms, 126 ms and 361 ms, respectively. The relative size of the water pools, but not the relaxation times, depended on the frozen storage temperature and on the chilled storage period.

Effect of storage conditions on differential scanning calorimetry profiles from thawed cod muscle

Differential scanning calorimetry (DSC) was used for measuring thermal denaturation (30–80°C) in cod fillets stored at −20°C and −30°C for up to 12 months and subsequently from 0 to 21 days in modified atmosphere at +2°C. The denaturation profiles were subjected to multivariate data analysis in order to extract the effects of the various storage conditions. The main effect of the storage conditions was the difference in the denaturation profiles between short time chill storage for all samples and long time chill storage for samples stored at −30°C. There was also a smaller effect due to the difference between short time frozen storage and long time frozen storage for samples stored at −20°C. The variation in the thermograms was primarily due to changes in size and position of the myosin, sarcoplasmatic protein and actin denaturation peaks, where variation in position could be related to changes in pH. Frozen storage at both temperatures and chill storage for samples previously stored at −30°C caused a decrease in the myosin denaturation peak. The relationship between the denaturation profiles and the water distribution (determined by low-field NMR relaxation) was found to involve mainly the two fastest relaxing populations (named III and IV), where IV is expected to contain the water most tightly bound to protein. The amount of water in pool III and IV appeared to be directly correlated to the changes in the denaturation profile during storage: the amount of MIV increased and MIII decreased when myosin and sarcoplasmatic proteins denatured and the actin denaturation peak shifted to a lower temperature.

Low-temperature transitions in cod and tuna determined by differential scanning calorimetry

Differential scanning calorimetry measurements have revealed different thermal transitions in cod and tuna samples. Transition temperatures detected at −11°C, −15°C and −21°C were highly dependent on the annealing temperature. In tuna muscle an additional transition was observed at −72°C. This transition appeared differently than the thermal events observed at higher temperatures, as it spanned a broad temperature interval of 25°C. The transition was comparable to low-temperature glass transitions reported in protein-rich systems. No transition at this low temperature was detected in cod samples. The transitions observed at higher temperatures (−11°C to −21°C) may possibly stem from a glassy matrix containing muscle proteins. However, the presence of a glass transition at −11°C was in disagreement with the low storage stability at −18°C during practical time scales. It was proposed that freezing of cod could be associated with more than one glass transition, with a glass transition at a temperature lower than −11°C being too small to be detectable with instrument, yet governing important deterioration processes. In order to optimize frozen storage conditions, the relationship between deterioration processes important for preservation of quality and glass transition temperatures still needs to be established.

Changes in liquid-holding capacity, water distribution and microstructure during chill storage of smoked salmon.

BACKGROUND During recent years, increases in liquid loss and soft texture in cold smoked salmon have been reported, possibly due to increased lipid content and a change in muscle structure. These problems can lead to downgrading of the product. This study aimed to investigate the influence of raw material composition and chill storage on quality parameters of cold smoked salmon. RESULTS Initial smoked fish had a higher liquid-holding capacity (LHC) than samples stored for 20 days. Large fish lost more liquid than small fish. The difference in LHC was consistent with a change in water distribution, which could indicate denaturation of muscle protein. Studies of the microstructure showed the influence of both processing and chill storage. An indication of lipid released from the cells was seen after 20 days of chill storage, which could be related to the reduced LHC at that time. CONCLUSION Both raw material composition and chill storage affected the quality parameters of smoked salmon. This study has improved knowledge about the relationships between muscle structure, liquid-holding properties and water distribution in smoked salmon. Copyright